1. Field of the Invention
This invention relates to a radio tele-communication device used in a mobile radio tele-communication system such as a portable type car radio telephone system, cordless radio telephone system or paging system and more particularly to a radio tele-communication device having a variable gain controlling function for stabilizing the amplitude level of a received signal.
2. Description of the Related Art
In a mobile radio tele-communication device, the level of a received signal may be varied by the influence of fading and the gain of a circuit may be varied by the environmental variation or the like. Therefore, in general, a variable gain control circuit is provided in the reception circuit system of the mobile radio device.
The reception circuit system of the mobile radio device having the variable gain control circuit provided therein is constructed as follows, for example. That is, a high-frequency signal received by an antenna is converted into an intermediate frequency signal by means of a frequency conversion circuit and then the amplitude level thereof is variably controlled by a variable gain amplifier. A signal output from the variable gain amplifier is converted into a digital signal by an analog/digital (A/D) converter so that it can be input to a digital demodulation circuit. In the digital demodulation circuit, the operation of demodulating a digital intermediate frequency signal input from the A/D converter into a base band signal is effected. Part of the demodulated base band signal is input to a level detection circuit. In the level detection circuit, the amplitude level of the base band signal is detected. A detection signal of the detected amplitude level output from the level detection circuit is converted into an analog signal by a digital/analog (D/A) converter. The thus converted detection signal is supplied as a gain control signal to the variable gain amplifier. The gain of the variable gain amplifier is variably controlled by the gain control signal.
However, in the above conventional variable gain control circuit, the D/A converter is used to derive the gain control signal. Therefore, the circuit scale of the reception circuit system is enlarged, the power consumption is increased and the number of pins used when the circuit is formed in a large integrated circuit (LSI) configuration is increased so as to increase the size of the radio communication device. The problems are extremely undesirable in a portable type mobile radio communication device using a battery as a power source since the portability thereof is lowered and the service life of the battery is shortened.